The art of lithographic printing is based upon the immiscibility of oil and water, wherein an oily material or ink is preferentially retained by an imaged area and the water or fountain solution is preferentially retained by the non-imaged areas. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-imaged areas retain the water and repel the ink while the imaged areas accept the ink and repel the water. The ink is eventually transferred to the surface of a suitable substrate, such as cloth, paper or metal, thereby reproducing the image.
Very common lithographic printing plates include a metal or polymer support having thereon an imaging layer sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.
Thermally sensitive printing plates are less common. Examples of such plates are described in U.S. Pat. No. 5,372,915 (Haley et al). They include an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation absorbing compound. While these plates can be imaged using lasers and digital information, they require wet processing using alkaline developer solutions.
Dry planography, or waterless printing, is well known in the art of lithographic offset printing and provides several advantages over conventional offset printing. Dry planography is particularly advantageous for short run and on-press applications. It simplifies press design by eliminating the fountain solution and aqueous delivery train. Careful ink water balance is unnecessary, thus reducing rollup time and material waste. Silicone rubbers such as poly(dimethylsiloxane) and other derivatives of poly(siloxanes)! have long been recognized as preferred waterless-ink repelling materials.
It has been recognized that a lithographic printing plate could be created containing an IR absorbing layer. Canadian 1,050,805 (Eames) discloses a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer comprised of laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose) and an optional cross-linkable resin. Such plates were exposed to focused near IR radiation with a Nd.sup.++ YAG laser. The absorbing layer converted the infrared energy to heat thus partially loosening, vaporizing or ablating the absorber layer and the overlying silicone rubber. The plate was developed by applying naphtha solvent to remove debris from the exposed image areas. Similar plates are described in Research Disclosure 19201, 1980 as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoated layer. These plates were developed by wetting with hexane and rubbing. CO.sub.2 lasers are described for ablation of silicone layers by Nechiporenko & Markova, PrePrint 15th International IARIGAI Conference, June, 1979, Lillehammer, Norway, Pira Abstract 02-79-02834. Typically, such printing plates require at least two layers on a support, one or more being formed of ablatable materials.
While the noted printing plates used for digital, processless printing have a number of advantages over the more conventional photosensitive printing plates, there are a number of disadvantages with their use. The process of ablation creates debris and vaporized materials that must be collected. The laser power required for ablation can be considerably high, and the components of such printing plates may be expensive, difficult to coat, or unacceptable in resulting printing quality. Typically, such printing plates require at least two layers on a support, one or more being formed of ablatable materials.
Some thermally switchable polymers have been described for use as imaging materials in printing plates. By "switchable" is meant that the polymer is irreversibly rendered either more hydrophobic or hydrophilic upon exposure to heat.
As an alternative method of preparing printing plates, U.S. Pat. No. 4,634,659 (Esumi et al) describes imagewise irradiating hydrophobic polymer coatings to render exposed regions more hydrophilic in nature. While this concept was one of the early applications of converting surface characteristics in printing plates, it has the disadvantages of requiring long UV light exposure times (up to 60 minutes), and the plate's use is in a positive-working mode only.
JP Kokai 95-023030 describes a printing plate having a hydrophilic surface layer and an imaging layer containing a copolymer prepared from isobutylene maleic anhydride. An argon laser is used for imaging, and the unexposed regions are washed away with ethanol. It would be desirable to avoid such wet processing conditions.
EP-A 0 652 483 (Ellis et al) describes lithographic printing plates imageable using IR lasers, and which do not require wet processing. These plates comprises an imaging layer that becomes more hydrophilic upon the imagewise exposure to heat. This coating contains a polymer having pendant groups (such as t-alkyl carboxylates). One problem with such materials is that they may be difficult to manufacture, exhibit poor shelf life, require a photoacid generator for imaging, and are positive-working only.
Thus, the graphic arts industry is seeking alternative means for providing a processless, direct-write, negative-working lithographic printing plate that can be imaged without ablation and the accompanying problems noted above. It would also be desirable to use "switchable" polymers without the need for processing after imaging, to render an imaging surface more hydrophobic in exposed areas, yet more hydrophilic in unexposed areas.